Conventional thin display devices display images by adjusting the light leaking from the optical waveguide by controlling the contact and separation of the displacement transmission unit with and from the optical waveguide using a piezoelectric actuator (see Japanese Laid-Open Patent Publication Nos. H7-287176, H10-78549 and H11-73142). Also, there are conventionally proposed systems that leaked evanescent waves by bringing a reflection prism, an extraction unit driven by static electricity, close to the optical waveguide (see Japanese Laid-Open Patent Publication Nos. 2000-75223, 2000-258701 and 2000-330040).
Also an optical switch has been proposed by this inventor that leaks guided light by attracting a cantilever film to the optical waveguide by electrostatic drive (see Japanese Laid-Open Patent Publication Nos. 2001-304537 and 2002-029594).
Additionally, there are conventionally used display units that emit the light, as pixels, from a back light through a filter of three primary colors of light and a polarizer using liquid crystal as the optical shutter. Other conventionally used display devices display pixels by scattering the guided light reflected at the intersurfaces of the glass plates using light-scattering liquid crystal. Liquid crystal is sealed between two glass plates to comprise the optical waveguide (see Japanese Laid-Open Patent Publication No. H11-109349).
Also available were display devices capable of full-color display of light in three primary colors, which is propagated through an optical waveguide, through changes in the refractive index at an optical output unit provided in the optical waveguide (see Japanese Laid-Open Patent Publication 2000-172197 and U.S. Pat. No. 6,236,799).
However, a problem with the conventional displacement transmission unit driven by a piezoelectric actuator was the thick piezoelectric element needed to transmit a large displacement, resulting in large mass, requiring large drive power, and producing slow response. The system with the reflection prism, an electrostatic extraction unit, located close to the optical waveguide has a problem in that its reflection prism has to have a large mass and is unable to provide high-speed response. Further, the electrodes to drive the extraction unit by static electricity are almost parallel to each other and are located at a distance through a support, thus requiring large voltage for attraction. Also, the optical switch to leak the guided light by attracting the cantilever thin film to the optical waveguide by static electricity has a problem in that it needs high voltage for attraction because the cantilever film is isolated from the optical waveguide. Additionally, the system that uses light leakage through the contact part of the above-mentioned optical waveguide and drive unit has its problems. It is unreliable due to fatigue of its mechanical drive unit, has unexpected scattering of the guided light due to foreign matter such as dust sticking to the core of the optical waveguide which is exposed to the air, and its contact part must be a thick and heavy drive unit to ensure large leakage of the guided light and pixels with sufficient brightness. For the above reasons, the advent of reliable (with no mechanical drive unit if possible), bright, highly responsive, and less power-consuming display devices has been anticipated.
A conventional display system using liquid crystal as the optical shutter has the following problems. It is difficult to see clearly: it varies greatly depending on the viewing angle because the system emits the light from the back light through the liquid crystal and uses a polarizer. Further, it is difficult to produce in thin configurations because it has a complex structure comprising a color filter and polarizer, and thus has is also expensive.
A display system that seals the light-scattering liquid crystal between two glass plates as the optical waveguide is reliable because it has no mechanical drive unit, but its optical waveguide is two glass plates with liquid crystal held in between them. It has the following four problems.
First, because its optical waveguide is exposed, foreign matter such as dust sticks to its surface, causing the guided light to leak and degrade the quality of the image second, the two glass plates, which are used also as the optical waveguide, are the supporting materials and have to be thick. Thus, it is difficult to form a thin and precise optical waveguide. Third, instead of miniaturizing the optical waveguide, the light of the three primary colors is guided through the same optical waveguide, and each pixel is set to scattering mode synchronously with the light emission, so that the light of the three primary colors can use the liquid crystal pixel of one point for full-color display. This system needs only one third of the pixels that the conventional method needs. But, brightness is necessary to see an image. The shining of one pixel for an instant is too dark. Flickering is large unless the sequentially driven pixels emit light simultaneously. Thus, it is difficult to get a screen of high brightness with such a system because it uses only a transient residual image. Fourth, comprising two glass plates, the board is too hard to provide a flexible image display of the screen and is vulnerable to impact and damage.
The display device that scatters the light from the initially provided light output unit by changing the refractive index at the light output control unit provided in the optical waveguide and by guiding the guided light to the light control unit cannot provide a screen of the required brightness. This is because currently available liquid crystals cannot produce sufficient changes in the refractive index for only a pixel of about 100 É m to guide enough light to the light output control unit.